Material that recieves shadows but no shading
The closest I think I can get is to use a toon-ish style shader on the bottom plane. This leaves the plane flat-shaded, but allows for pure black shadows (remember to set the ColorRamp to &quot;constant&quot;). However, as you can see, the falloff from the point light also gets cast in black as well. This can be solved to an extent by increasing the size of the light source, but doing this causes the shadow resolution to suffer as well.
Here is an example using a point light:

However, if your mesh objects are sufficiently small (can be scaled down), this issue becomes negated:

As Markus pointed out in his comment, changing the light type to Sun fills out the plane (regardless of the size of your mesh), but the shadow quality properties will likely need to be increased to make the shadow look &quot;cleaner&quot; (less aliased), but it still may not come out perfect: